The present invention is in the field of rotational molding; more particularly, the invention relates to rotationally molded articles and a method to rotationally mold articles having at least two layer made of different polymeric materials.
Rotational molding, also known as rotomolding, is used in the manufacture of hollow objects from thermoplastics. In the basic process of rotational molding, solid or liquid polymers are place in a mold. The mold is first heated and then cooled while being rotated about two perpendicular axes simultaneously. Many polymers can be used in the rotational molding process. Popular polymers for this use are polyolefins such as polyethylene. It is also known to use polycarbonates, crosslinkable polyethylene, nylon, and other materials. In selecting the rotational molding grade formulation, care must be taken to assure that there will not be thermal degradation during the heating cycle. A general discussion on rotational molding is given in MODERN PLASTICS ENCYCLOPEDIA 1979-1980, Volume 56, No. 10A, beginning at Page 381.
U.S. Pat. No. 3,542,912 discloses rotational molding of multi-layered articles. In this disclosure the rotationally molded article has layers of different polymers in the article wall. The outside layers have a higher melting point than the inside layers.
U.S. Ser No. 441,820 filed the same date as this application, entitled Rotational Molding of Articles Having Impact Resistance by Steinberg and Flood discloses a composition useful for rotational molding. This composition comprises a polyamide, preferably polyepsiloncaprolactam, on 1 to 15, preferably 3 to 10 percent by weight of a copolymer of an alphaolefin, preferably ethylene and a carboxylic acid, preferably methacrylic acid, neutralized up to 100% with metal, preferably zinc ions. This patent application is hereby incorporated by reference.